Formally embarking on a second term, Vice President Joe Biden took the oath of office Sunday, surrounded by family and friends in an early morning ceremony that kicked off a day of celebrations marking four more years for the Obama administration.

. Vice President Joe Biden (2nd L) takes the oath of office from U.S. Supreme Court Justice Sonia Sotomayor (2nd R) as his wife Dr. Jill Biden (R) looks on during the official swearing-in ceremony at the Naval Observatory (Saul Loeb-Pool/Getty Images)

President Barack Obama was to be sworn in just before noon at the White House, 24 hours before re-enacting the ceremony before an expected crowd of hundreds of thousands gathered at the Capitol and across the National Mall.

Biden, following a private Mass, was sworn in at the Naval Observatory. Justice Sonia Sotomayor, appointed by Obama as the first Hispanic on the Supreme Court, administered the oath of office to Biden, who placed his hand on a Bible his family has used since 1893.

"I will support and defend the Constitution of the United states," Biden said as he recited the oath.

Among the 120 guests on hand to witness the vice president's second swearing-in were Attorney General Eric Holder, departing Interior Secretary Ken Salazar and several Democratic lawmakers.

Sunday's subdued swearing-in ceremonies are a function of the calendar and the Constitution, which says presidents automatically begin their new terms at noon on Jan. 20. Because that date fell this year on a Sunday — a day on which inaugural ceremonies historically are not held — organizers scheduled a second, public swearing-in for Monday.

Obama and Biden were also to lay at a wreath at Arlington National Cemetery Sunday morning, then address supporters at an evening reception.